Photocathodes

ABSTRACT

A method of manufacturing a photocathode for an electron tube whose active constituent is formed by a strongly P-conductive AIII-BV compound or mixed crystals of such compounds in which cesium is deposited on the surface of the cathode until after having reached a maximum the photosensitivity drops to a value lying between 10 percent and 50 percent that of the maximum. Thereafter, the surface of the cesium is exposed to oxygen to form Cs20 and these steps are repeated until an activating layer about 50-100 A. in thickness is formed.

United States Patent 1,994,904 3/1935 Wilson 1 17/224 X 2,070,691 2/1937 Spencer... 1 17/224 X 2,190,695 2/1940 Bruining.. 1 17/219 X 2,228,945 l/ 1941 Bruining 1 17/224 X 2,698,397 12/1954 befcount 117/224 X 2,702,259 2/1958 Sommer 1 17/107 X 2,739,084 3/1956 Sommer 117/219 X 2,880,344 3/1959 Stovdenheimer 1 17/107 X Primary Examiner-Alfred L. Leavitt Assistant ExaminerAlan Grimaldi Att0rneyFrank R. Trifari [72] Inventor Andrew Alfred Turnbull London, England [21] Appl. No. 722,864 [22] Filed Apr. 22, 1968 [45] Patented Jan. 4, 1972 [73] Assignee U.S. Philips Corporation New York, N.Y. 32 Priority Apr. 21, 1967 [33] Great Britain [31] 18,486/67 [54] PHOTOCATHODES 4 Claims, 4 Drawing Figs.

[52] US. Cl 117/219, 117/107,117/224, 313/94, 313/346 R [51] Int. Cl B44d l/l8, B0 1 j 39/00 [50] Field ofSearch 117/224, 107, 202, 219; 313/94, 346

[56] References Clted UNITED STATES PATENTS 3,387,161 6/1968 Van Laaret a1. 313/94 PATENTED JAN 4m 316132.442

FIG.2.

FIG/I.

INVENTOR ANDRE A- TURNBILL BY A M 2- PHOTOCATHODES Method of manufacturing an electric discharge tube comprising a photocathode and electric discharge tube manufactured by said method.

The invention relates to a method of manufacturing an electric discharge tube comprising a photocathode, the active constituent of which is formed by a strongly P-conductive AIll-BV compound or mixed crystals of such compounds which are activated by means of an alkali metal. The invention furthermore relates to an electric discharge tube comprising a photocathode manufactured by said method.

It is customary to denote by AIII-BV a compound which is a substantially equiatomic intermetallic compound of one of the elements Alll of the third column of the Periodical System, boron, aluminum, gallium, indium, and an element BV of the fifth column of the Periodical System, nitrogen, phosphorus, arsenic, antimony. With mixed crystals the quantity of AIII atoms is approximately equal to that of the EV atoms.

Photocathodes of the above compositions are described by Scheer and Van Laar in Solid State Communications, US. Pat. No. 3,387,161, 1965 and in Dutch Patent application 64-13961.

With the known photocathodes the activating layer consists substantially of a monoatomic layer of cesium, as the case may be with oxygen.

It has been found, however, that such cathodes are very sensitive to small quantities of impurities on the surface of the semiconductor. Therefore, the requirements with respect to the vacuum are very severe. On the other hand such photocathodes exhibit, after a comparatively short period of photoemission, a drastic drop of sensitivity.

The invention has for its object to provide a cathode and a method in which the requirements relative to vacuum are less extreme and a decline of sensitivity does not occur so soon.

In a method of manufacturing an electric discharge tube having a photocathode whose active constituent is formed by a strongly P-conductive AIII-BV compound or mixed crystals of such compounds which are activated by means of an alkali metal, such a quantity of alkali metal is admitted, in accordance with the invention, to the surface of the semiconductor that after having reached a maximum the photosensitivity drops to a value lying between 50 and percent of the maximum, after which such a quantity of oxygen is admitted that the photosensitivity again reaches a maximum, which treatments with the alkali metal and the oxygen are repeated once or several times.

It has been found that the photocathodes thus manufactured are less sensitive to impurities and that even semiconductor crystals having been in contact with air are activated to a satisfactory sensitivity. The drop of sensitivity does not occur as soon as in the known photocathodes.

The advantageous effect of the method according to the invention is probably due to the fact that more alkali metal is admitted than the quantity corresponding to a monolayer so that the impurity affects the larger quantity of alkali metal to a lesser extent, while the larger quantity of alkali metal supplied to the cathode provides at the same time a larger quantity of alkali metal for the interior of the discharge tube. Since the alkali metals have fairly high vapor pressures, transport phenomena of the alkali metals may readily occur in such discharge tubes, but in the presence of a larger quantity on the cathode than that of a monolayer, they are less likely to give rise to modifications of the photocathode structure.

By the method according to the invention one or two monolayers of the alkali metal will be applied during each treatment, and preferably not more than five treatments are carried out, the thickness of the layer being about 50 to 100 A. According to the invention the treatment is carried out in oxygen at a pressure of 5X10 Torr so that in the case of cesium as the alkali metal the composition of the layer corresponds approximately to the formula Cs 0.

Although the influence of impurities on the cathode surface is also reduced by the method according to the invention, it may be advisable according to the invention in the event of strong contamination, for example, due to a long stay in air, to sinter the semiconductor surface in alkali metal vapor at a temperature of a few hundred degrees for a few minutes. In the case of cesium this temperature is 300 C.

The invention will be described more fully with reference to the drawing, in which FIG. I shows a contaminated semiconductor surface with an activating layer, and

FIGS. 2 to 4 shows discharge tubes comprising a photocathode according to the invention.

Referring to FIG. 1, reference numeral 1 designates part of a gallium-arsenide crystal having a dopant of 4X10 zinc atoms per cc. The surface of the crystal is slightly contaminated by a layer 2 mainly of oxygen, shown on an exaggerated scale. The activating layer 3 consists of cesium oxygen of 50 A. thick having a work function of 1.3 ev., which is slightly lower than the energy gap of 1.4 ev. of the gallium-arsenide crystal.

The photocathode formed by the crystal and the activating layer are worked as follows. The crystal 1 having an upper surface obtained by splitting and having (110) orientation is slightly contaminated by this treatment by the layer 2. The crystal 1 with the impurity 2 is exposed to cesium vapor until the photoemission for white light attains a maximum as deter mined in the manner disclosed in US Pat. No. 3,387,161, which issued June 4, 1968. Then the cesium supply is continued until the photoemission drops to about 15 percent of the maximum. Subsequently oxygen is admitted at a pressure of 10 Torr until the photoemission again attains a maximum. This treatment with cesium and oxygen is repeated once or several times, but not more than five times in total; then photosensitivities are obtained which are percent or more of that of the known photocathodes with monolayers, whereas they have a higher stability.

Suppose the impurity layer 2 is more serious than that which guarantees a satisfactory result, the cathode is heated prior to the alternating cesium and oxygen treatments in cesium vapor at 300 C. so that the impurity disappears approximately completely.

As is shown in FIG. 2, illustrating a photocell with a bulb 7, a gallium-arsenide layer is deposited from the vapor phase on a molybdenum plate 8, which layer is treated in the same manner as the crystal 1 of FIG. I. The anode is designated by 10.

FIG. 3 shows a photoelectron multiplier tube in which the vapor-deposited cathode layer 12 of gallium-arsenide is applied to the flat window 11. Also this cathode is treated in the manner described with reference to FIG. 1. The multiplier electrode 13 and the anode 14 form the further electrodes of the tube.

As is shown in FIG. 4, a gallium-arsenide layer 16 is applied to the flat window 15 of the iconoscope, which layer is also activated by an alternating treatment with cesium and oxygen. Of the further electrodes of the tube only the filament cathode 17 is shown.

What is claimed:

1. A method of manufacturing an electric discharge tube having a photocathode whose active constituent is formed by a strongly P-conductive AIII-BV compound or mixed crystals of such compounds which are activated by means of an alkali metal, comprising the steps of depositing a strongly P-conductive AIIl-BV compound or mixed crystals of such compounds onto the surface of the photocathode substrate, then depositing a quantity of alkali metal onto the surface of the P-conductive AIII-BV compound or mixed crystals until after having attained a maximum the photosensitivity drops to a value lying between 50 and 10 percent of that of the maximum, thereafter adding to the cesium a quantity of oxygen until the photosensitivity again reaches the maximum, and repeating the deposition of the alkali metal until the sensitivity reaches a maximum and adding the oxygen at least once.

2. A method as claimed in claim I wherein in each treatment with the alkali metal one to two monolayers are deposited from the vapor phase.

3. A method as claimed in claim 1 wherein the addition of oxygen is carried out at a pressure of about 5X10 Torr.

4. A method as claimed in claim 1, wherein prior to the deposition of alkali metal on the semiconductor surface, the

surface is heated in alkali metal vapor at about 300 C. S 

2. A method as claimed in claim 1 wherein in each treatment with the alkali metal one to two monolayers are deposited from the vapor phase.
 3. A method as claimed in claim 1 wherein the addition of oxygen is carried out at a pressure of about 5 X 10 6 Torr.
 4. A method as claimed in claim 1, wherein prior to the deposition of alkali metal on the semiconductor surface, the surface is heated in alkali metal vapor at about 300* C. 